1. Field of the Invention
This invention relates to a method for manufacturing a shadow mask which is used in, for example, color a television picture tube.
2. Description of the Related Art
FIG. 1 of the accompanying drawings is a sectional view showing primary portions of a color television picture tube. Electronic beams 1, 2 and 3, corresponding to the red, green, and blue colors respectively, and emitted from an electronic gun, pass through a number of fine apertures 5 which are regularly arranged color selection electrodes of the shadow mask 4. Thereafter, the electronic beams 1, 2, and 3 correctly collide against the corresponding fluorescent materials 6, 7 and 8, which render luminous phenomenon of red, green and blue colors respectively, of a fluorescent screen 9 formed at the inner surface of a panel 10, so as to present color images.
As a material of the shadow mask 4 in such a color picture tube, a low carbon Al killed steel, containing high purity Fe as a main component, has been generally used heretofore. This selection is made by integrally taking the machinability, strength, cost and the like into consideration.
Despite having excellent machinability, such a conventional shadow mask 4 for color picture tube has been disadvantageous in that its color purity tends to degrade due to the phenomenon called doming; in operation of a color picture tube, generally only 1/3 of the total electronic beam pass through the aperture 5 of the shadow mask 4. The residuals collide against the shadow mask 4 itself, not against the fluorescent screen, thereby causing the shadow mask 4 to be heated over 80.degree. C. As a result, the shadow mask 4 becomes thermally expanded and stressed, impeding the electronic beams from correctly colliding against the fluorescent screen. Thus, the color purity degrades. The thermal expansion coefficient of the Al killed steel, used as the raw material for the shadow mask, is large, being 1.2 * 10.sup.-5 /deg at 0.degree.-100.degree. C. This has been a serious problem in shadow masks which are to be evolved toward higher refinement.
To cope with the problem mentioned above, a shadow mask made of e.g. Fe-Ni type invar alloy (Fe-Ni 36%) having smaller thermal expansion coefficient than in Al killed steel has been conventionally used, as described for example in Japanese Patent Laid-Open No. 25446/1967, 58977/1975, or 68650/1975.
However, the shadow mask made of invar alloy is inferior in aseismatic property to a shadow mask made of Al killed steel. This inferiority is mainly attributable to the lowering of Young's modulus of the shadow mask itself, caused by raw material properties and the high temperature annealing process executed to improve the shadow mask's formability.
Namely, in addition to the original lowness in Young's modulus of invar alloy being 1400 kgf/mm.sup.2 in comparison with that of the conventional Al killed steel being 20000 kgf/mm.sup.2, the high temperature annealing for improving the formability of the shadow mask acts to make its crystal grain bulky, thereby further lowering the Young's modulus.
This lowering of the Young's modulus reduces the resonance frequency and causes so-called howling, a phenomenon that the shadow mask itself resonates and trembles by external vibrations of sounds from the speaker etc. when incorporated into the color picture tube. Consequently, there would arise a positional divergence between the aperture of the shadow mask and the electronic beam, so as to degrade the color purity. This has been a serious obstacle to put the shadow mask to practical use, meeting the recent strong requirement of higher refinement.